I’m sure I’m not alone in having a few rings on my desk from mugs of coffee, but I never thought of the stains as being table top chromatography before. However, American chemists at the University of California have used the idea to perform size based particle separation.

As liquid evaporates, suspended particles concentrate on the rim of a dried droplet. This is why the edges of a coffee ring stain are darker. But these scientists have shown that as a droplet dries on a glass slide, it also separates nanometre sized particles with the smaller particles closer to the edge of the droplet. The particles stop where their diameter matches the height of the liquid at that point.

And this isn’t just theory, the technique has separated antibodies and bacterial and mammalian cells, showing that coffee ring chromatography could be a low cost way to carry out particle separations without electricity.